Optical fibers are currently used to transmit optical signals. Optical fibers, including multimode optical fibers, are frequently used for data transmission or high-speed data transmission over distances ranging from a meter or less up to the distance needed to transmit throughout a building or between buildings near one another that are optical signals associated with local networks.
Multimode fibers, by definition, are designed to support multiple guided modes at a given wavelength. The bandwidth of a multimode fiber is defined by the fiber's ability to carry the different optical (guided) modes with little or no temporal separation as they travel down the fiber. This requires that the group velocities of the different optical modes be as close to the same value as possible. That is to say, there should be minimal intermodal dispersion (i.e., the difference in the group velocity between the different guided modes should be minimized) at the design (“peak”) wavelength λP.
A multimode optical fiber can be designed to minimize the amount of intermodal dispersion and differential delays between mode groups. This is done by providing the core of the multimode fiber with a gradient-refractive-index profile whose shape is generally parabolic. The gradient-index profile is optimized for reducing intermodal dispersion when the additional distance traveled by higher-order modes is compensated for by those modes seeing a lower refractive index than lower-order modes that have to travel a shorter distance, the result being that all modes travel substantially the same overall optical path. Here, optical path means the physical distance traveled multiplied by the index of refraction of the material through which the light travels.
This minimization of intramodal dispersion becomes complicated when the light source used to send light down the multimode fiber is not strictly monochromatic. For example, a vertical-cavity, surface-emitting laser (VCSEL) has a wide-spectrum discrete emission. The VCSELs used for high-speed data transmission applications are generally longitudinally, but not transversally, single mode. As it turns out, each transverse mode of a VCSEL has its own wavelength corresponding to the various peaks of the emission spectrum, with the shorter wavelengths corresponding to the higher-order modes. Accordingly, a multimode fiber that is optimized to have a maximum bandwidth for a given wavelength will not exhibit optimum bandwidth performance when the light source causes the different modes to have different wavelengths.
Variations in the intramodal dispersion can also occur when the peak wavelength λP of the multimode fiber does not coincide with the operating wavelength, λO. For example, small errors in the curvature (alpha) of the core can result in λP values that are lower or higher than the target value, and this results in lower bandwidth due to variations in the optical path lengths for the different propagating mode groups. This situation can also arise when signals at more than one wavelength propagate in the multimode fiber, for example with coarse wavelength division multiplexing (CWDM). Signals propagating at lower or higher wavelengths than λP will incur larger differential delays than desirable, thereby decreasing the bandwidth and degrading the system performance.
One solution to the problem is to form the multimode fiber with a refractive-index profile that provides an optimized bandwidth for a light source having a particular transverse polychromatic mode spectrum rather than a single wavelength. Such an approach is described in U.S. Pat. No. 7,995,888 (hereinafter, the '888 patent). This approach makes sense under the assumption that light sources such as VCSELs all have generally identical wavelength spectra. However, the polychromatic mode spectra for VCSELs can differ substantially between the same types of VCSELs, as well as between different types of VCSELs. This means that a different optimized multimode optical fiber would have to be designed to match each of the different possible polychromatic mode spectra for VCSELs used in telecommunications applications. This approach is inefficient, and from a commercial telecommunications viewpoint is impractical and expensive to implement.